1. Field of the Invention
The present invention relates to an imaging apparatus such as an X-ray imaging apparatus for taking an X-ray image of an object to be imaged by using an X-ray and displaying the imaged X-ray image as a visual image on a display monitor.
2. Description of the Background Art
A conventional example of an X-ray imaging apparatus is shown in FIG. 1. This X-ray imaging apparatus 100, which is to be incorporated into an X-ray diagnostic apparatus, comprises: an imaging unit 130 including an X-ray tube 110 for irradiating an X-ray onto a patient P to be imaged, an image intensifier 101 for converting an X-ray image formed by the X-ray irradiated from the X-ray tube 110 through the patient P into an optical image, and an optical system 104 for optically processing the optical image obtained by the image intensifier 101; an imaging tube 102 for converting the optically processed optical image from the imaging unit 130 into electric image information signals; a signal processing circuit 105 for processing the image information signals from the imaging tube 102 by applying processings such as a noise suppression processing and an A/D input matching processing; an A/D converter 103 for converting the processed image information signals from the signal processing circuit 105 into digital image signals; a memory 106 for memorizing the digital image signals obtained by the A/D converter 103; a D/A converter 107 for converting the digital image signals memorized by the memory 106 into analog image signals; and a monitor 120 for displaying an image of the patient P given by the analog image signals obtained by the D/A converter 107.
Here, the imaging tube 102, the signal processing circuit 105, the A/D converter 103, the memory 106, and the D/A converter 107 are usually contained inside a TV camera unit (not shown).
Now, the fastest A/D converter available at a time of this application is the 12 bit, 10 MHz A/D converter, so that the maximum imaging rate for obtaining an image of 1000.times.1000 image elements with a concentration levels given by 12 bit in a digital fluorography apparatus such as an X-ray imaging apparatus described above is limited to 7.5 frames/sec. In other words, in order to speed up this imaging rate from 7.5 frames/sec to 30 frames/sec, it is necessary to have an A/D converter having a processing speed four times faster than the fastest A/D converter currently available, but since such a high speed A/D converter is not available yet, there has been no prospect conventionally for speeding up the imaging rate in the digital fluorography apparatus.
In addition, there has conventionally been a problem that as the imaging rate is increased by reading out the image data at higher speed, the dynamic range of the image data become lower.